1. Field of the Invention
This invention relates to the field of overcoming drag caused by the relative underwater flow of fluid past a cable. Specifically, this invention relates to the field of using improved tow cables to better control a submerged object.
2. Description of the Related Art
This invention relates to the towing of submerged objects, commonly called "fish." These fish can be sonar devices, deep-sea exploration vehicles, or other underwater vehicles that are towed underwater. These fish are often towed behind a towing vehicle, such as a ship or submarine. In addition, they can be tethered to a stationary object. The typical arrangement is for the towing vehicle to be a ship, which will be attached to the fish by a cable. In order to submerge the fish, the cable will be played out until the fish sinks to the desired depth.
The foregoing described arrangement is generally satisfactory either where a ship is moving relatively slowly, or where the current is minimal, or where the cable length is relatively short. However, depending on both the relative speed of the water flowing past the cable and the length of the cable, this arrangement can result in significant drag produced by the water on the cable. Because of such increased drag, more cable is required to maintain the fish at a given depth. As the length of the cable is increased, the weight of the entire towing apparatus increases. Furthermore, as the length of the cable increases, the operator's ability to control the fish decreases. Thus there has been a long felt need to find a way to reduce the effect of this drag in order to both reduce the amount of cable used, and to increase the operator's control over the fish at a desired depth.
To date, the prior art has focused on attempts to passively reduce drag on the cable, which generally consisted of improved fairing shapes. These fairings are airfoil-shaped coverings that are designed to streamline the profile of the cable in order to reduce drag on the cable. There are many types of such fairings. Examples are disclosed in U.S. Pat. No. 5,050,445, which describes a fairing that completely covers the cable, and in U.S. Pat. No. 4,829,929, which describe a fairing that only partially covers the cable. In a variation on the fairing system, systems utilizing ribbons to additionally reduce drag are shown in U.S. Pat. No. 4,843,996. Lastly, where a fish requires the use of electricity, other cables were designed that enclose both the cable and the electrical lines. Examples of the latter referred to systems are disclosed in U.S. Pat. Nos. 3,379,161 and 3,343,516. While these systems typically did reduce drag, they were unable to eliminate it totally since such systems all lacked the capacity to produce thrust. Since the production of thrust is the only way to truly overcome drag, such attempts to passively reduce drag prove only to be partially effective.
As previously noted, where cable lengths are lengthened, the capability to control the fish became more difficult. Since certain towing applications required greater control over the fish, attempts were made to devise systems that provided such control. The systems shown in U.S. Pat. Nos. 3,987,745 and 4,843,996 dealt with this problem by creating two fish: one that maintained a general base position, and a second that could explore out from the base position under its own power. However, such solution is not practicable in all towing situations.
Another technique has been largely confined to the field of towed hydrophone arrays where the cable needs to extend horizontally over great distances. In those situations, systems such as those disclosed in U.S. Pat. Nos. 3,605,674 and 4,290,124 use controllable wings attached to the cables. These wings maintain the cable horizontally at a predetermined depth as the entire array is towed. In other towing arrangements, such as that shown in U.S. Pat. No. 4,709,355, a closed loop feedback system is utilized where the controller is located on a ship and automatically maintains the wings at a desired angle to maintain or alter its depth based on sensor readings. However, such technology was never applied to the cable fairings used in towing fish, since without some means of providing thrust to the cable there was no way to correct a fairing segment to keep it in desired alignment.
Thus, prior to the present invention, there was no active means to overcome the drag on cable fairings, and no effective way to control the cable, resulting in the use of longer and thicker cables than those utilized in the system of the present invention.